Trigeminal ganglion development is a well studied model of vertebrate neurogenesis, where neural crest and placode cells come together to form this group of sensory neurons. Several genes are known to be expressed in cells contributing to the ganglion, but the function of most has not been elucidated. This proposal aims to characterize the function of key gene products expressed during the early stages of trigeminal placode and ganglion development. Research proposed here will focus on two genes expressed in neurogenic placode cells: Pax-3 and FGFR4. Early in their development, placode cells express Pax-3 as they become committed. FGFR4 expression peaks during placode cell delamination. Several other genes are subsequently expressed in the trigeminal placode, including Brn3a, neurogenin-2, and other downstream neuronal markers. A model is proposed whereby these genes govern the stepwise process of placode development. The specific aims for this research are 1) determine the function of Pax-3 in trigeminal placode cell development, and 2) determine the function of FGFR4 in trigeminal placode cell development. Pax-3 and FGFR4 have been well studied in other embryonic model systems, providing a solid basis for experimental design. The results obtained from this research will be especially useful to neuroscientists, developmental and cancer biologist. The methods and experiments outlined in this proposal are based on extensive research showing that misexpression of genes in cells, tissues and live embryos is an effective approach to understanding cellular gene function. Preliminary data show the development and testing of misexpression and inhibitory constructs for these genes, which will be used in the chick embryo model system to fulfill the specific objectives. Proven techniques of gene misexpression will be utilized in this study, including RCAS retroviral mediated gene delivery and in ovo electroporation. The development of new reagents aimed at blocking gene expression including siRNA and morpholino technology have been successfully tested in our lab. Inhibition and misexpression experiments will be analyzed for cellular delamination, migration and proliferation, differentiation potential, and the expression of other important developmental genes. Important advances in our understanding of neuronal cell differentiation in development will be sought.